What a mess thought the Microbiologist. “So let me get this straight. You diagnosed community acquired pneumonia in your patient and started Amoxicillin and Clarithromycin. However on the ward round you felt it was more ‘severe’ than community acquired pneumonia so you switched the patient to the hospital acquired pneumonia antibiotics. The patient is no better, even after 4 days of these new antibiotics, so you want advice about how to escalate these antibiotics even further!?”

“Yes,” said the doctor.

“Okay, you need to put the patient back on to the antibiotics for community acquired pneumonia” replied the Microbiologist.

“But they’re really sick!” exclaimed the doctor.

“Yes and you are not actually helping them get better!!”…the Microbiologist went on to explain where it had all gone wrong…

​What is pneumonia?The standard definitions of pneumonia are:

Community acquired pneumonia (CAP) is an acute infection of lung tissue with onset outside of hospital or within 48 hours of admission to hospital.

Hospital acquired pneumonia (HAP) is a respiratory infection developing >48 hours after hospital admission and that was not incubating at the time of admission.

But do these definitions of CAP and HAP really matter in day-to-day clinical practice, surely they are both pneumonia? Well the answer is both yes and no… yes… the difference between CAP and HAP affects the way they are treated, but no… it’s not the 48 hour cut-off that matters! The real reason to distinguish between CAP and HAP is because the causative bacteria are different and therefore the antibiotic treatments are different. To understand this, we need to consider how pneumonia occurs and the common microorganisms that cause CAP and HAP.

How does pneumonia occur?Pneumonia is caused by bacteria which colonise the upper respiratory tract. Infection occurs when these bacteria get down in to the chest and our normal protective mechanisms fail to clear these bacteria e.g. the mucociliary escalator which removes sputum from the lungs, coughing out sputum (and bacteria) and our immune systems. If the bacteria remain in the lungs they set up an inflammatory process which causes tissue damage… pneumonia.

What are the common causes of CAP and HAP?Understanding how pneumonia occurs helps us understand why the causes of CAP and HAP are different. The bacterial flora of the upper respiratory tract of people in the community is different to people in hospital (see picture below). This flora changes at about day 4 of admission. The flora changes because of exposure to different microorganisms from the environment, physiological changes, immunosuppressants and selective pressure from antibiotics, hand washes and detergents.

Click for larger image

​The common causes of CAP and HAP are shown in the table. Whilst some bacteria cause both types of infection the important differences are highlighted in bold.

How do the different causative microorganisms change the antibiotics for CAP and HAP?For CAP treatment to be effective an antibiotic active against Mycoplasma pneumoniae, Legionella pneumophila and Chlamydophila pneumoniae is required. Therefore a macrolide such as Clarithromycin is added to a beta-lactam such as Amoxicillin or Co-amoxiclav.

For HAP treatment to be effective the antibiotics used must be able to cover Moraxella catarrhalis and Pseudomonas aeruginosa; the macrolide is not required as the bacteria it targets are not the common causes of HAP. The treatment of HAP usually involves a broad spectrum antibiotic such as Piptazobactam OR Meropenem or a combination of antibiotics such as Teicoplanin PLUS Ciprofloxacin.

There is a common myth in medicine that HAP is worse than CAP but this is simply not true; they can both be just as bad as each other. Remember, it is the causative bacteria of CAP and HAP that are different, not the severity of the condition; this is why the choice of antibiotic is different.

So where do nursing homes fit into this?Most antibiotic guidelines for HAP and CAP recommend treating all patients admitted to hospital from nursing homes as CAP but I don’t agree with this. I think the level of care the patient receives in the nursing home should be taken into account. Let me explain…

Some “nursing homes” are actually residential homes or sheltered accommodation. The residents have a safety net of someone able to come to their aid in an emergency e.g. falling down and unable to get back up, but otherwise they live independent lives. These residents will have the same normal upper respiratory tract flora as other people in the community and therefore I think it is entirely appropriate to treat someone admitted with pneumonia to hospital from such a home as having CAP.

However, other “nursing homes” provide levels of care similar to that which a patient receives in hospital. These residents may receive help with feeding, dressing, oral care, home oxygen therapy, lots of medications, hand washes and detergents etc. These residents are unlikely to have normal upper respiratory tract flora; it is more likely to be the same as patients in hospital. I therefore think if these residents develop pneumonia they should be treated as having HAP. I also think this is the case even if they are not actually admitted to hospital but remain in their nursing home setting under the care of their GP.

Okay so a patient admitted from a nursing home cannot by definition have hospital acquired pneumonia as it wasn’t acquired in a hospital, but maybe we should keep the acronym and change the meaning to “Healthcare Associated Pneumonia”…​I believe this would probably be more accurate.

So after a lengthy explanation to the doctor by the Microbiologist the patient was put back on the correct treatment for community acquired pneumonia and 48 hours later was feeling much better. They were discharged home the following day with oral antibiotics and made a full recovery.

Even though the chart shows some different bacteria that distinguishes the difference between the two pneumonias some are the same. How do you determine which one the patient has if they have the common bacteria of the two? Do you just follow the 48 hour rule or are there other guidelines?

David

23/8/2017 09:02:20 pm

Hi Virginia
Good question. The table shows the possible causes of both CAP and HAP which influences the initial choice of antibiotics in a sick patient to make sure all possible causes are covered. Samples (sputum, blood, urine for antigen testing) should then be sent to the laboratory to look for the specific cause and once this is known the antibiotics should be targeted at the cause. In the UK if we identify S. pneumoniae as the cause we would narrow down treatment to penicillin or if we identify M. pneumoniae then we might use Doxycycline. Once the cause is known it doesn’t matter whether it is CAP{ or HAP… you just treat for the cause.
Hope that clears that up and thanks for the question
David